1. Field of the Invention
The present invention relates to an optical pickup device in which a light beam is irradiated onto an optical recording medium to enable reading and writing of information, in particular, the present invention relates to structure of an optical pickup device which has a function to correct spherical aberration.
2. Description of Related Art
Optical recording media such as a compact disc (hereinafter referred to as a CD) and a digital versatile disc (hereinafter referred to as a DVD) have become commonplace and widely available. Further, researches to increase quantity of information recorded on the optical recording medium, have been carried on recently. As a result, optical recording media which can record large capacity of information such as a HD-DVD which is high definition DVD and a Blu-ray Disc (hereinafter referred to as a BD) are begun to put into practical use, for example.
Reading of information from the such optical recording medium and writing of information to such optical recording medium are performed utilizing an optical pickup device. Recently, developing of an optical pickup device by which reading and writing of information from/to a plurality of kinds of optical recording medium can be achieved, is performed actively because a plurality of kinds of the optical recording medium are used as above described.
By the way, these optical recording media such as a CD, a DVD, a BD and the like have transparent cover layers of different thickness to protect recording layers. For example, a thickness of the transparent cover layer in a CD is 1.2 mm, the thickness in a DVD is 0.6 mm, and the thickness in a BD is 0.1 mm. In the optical pickup device which has compatibility for the optical recording media that have different thickness of the transparent cover layers as above described, a problem is caused by generation of spherical aberration.
Further, in the optical recording media which especially have a target of high density recording such as a BD or the like, developing of an optical recording medium which has a plurality of recording layers in thickness direction of the optical recording medium is performed actively. When reading of information or the like is performed for the optical recording medium that has a plurality of recording layers utilizing an optical pickup device, a problem is also caused by generation of spherical aberration because thickness of the transparent cover layers are different depending on location of the recording layers (in this case an intermediate layer which is arranged between the recording layers is also counted as the transparent cover layer).
It should be noted that the problem of spherical aberration as above described becomes severe especially in an optical pickup device which utilizes a blue light source that requires an objective lens with large numerical aperture (NA). A demand for an optical pickup device in which the spherical aberration can be adequately corrected, becomes strong recently.
Because of these situations, developing of an optical pickup device which has a function to correct the spherical aberration has been heretofore performed actively. For example, there is an optical pickup device which is capable of correcting the spherical aberration by disposing a beam expander which makes diameter of an optical beam change in an optical system provided in the optical pickup device (for example, see background of the invention of JP-A-2006-147057). Further JP-A-2006-147057 proposes an optical pickup device in which correction of the spherical aberration is performed by disposing a liquid lens which can adjust a focal distance by adjusting applied voltage in an optical system provided in the optical pickup device and changing degree of divergence or degree of convergence of a light beam which is input to an objective lens.
Further JP-A-2005-071424 proposes an optical pickup device in which correction of the spherical aberration is performed by disposing a liquid crystal element in an optical system provided in the optical pickup device and applying voltage to the liquid crystal element such that a light beam that passes the liquid crystal element has arbitrary prescribed phase distribution.
However in case of the optical pickup device in which correction of the spherical aberration is performed utilizing the beam expander, because it has a structure to move a movable lens mechanically, it causes a problem that the optical pickup device tends to become large.
In this regard, in a case of the structure utilizing the liquid crystal element or the liquid lens, because the correction of the spherical aberration is performed by adjustment of the applied voltage, it has merit that the increase of device size can be suppressed in comparison with a case where correction of the spherical aberration is performed utilizing the beam expander. However, in the structure utilizing the liquid lens, it is difficult to perform fine tuned correction such that entire spherical aberration which is generated is made evenly low level in comparison with a case utilizing the liquid crystal element (for example, the liquid crystal element such as utilized in JP-A-2005-71424) in which the spherical aberration can be corrected by a plurality of arranged regions (phase shift regions) where phase of the input light beam can be adjusted. As a result, there is a tendency that a fine tuned correction of the spherical aberration is harder to be performed by the liquid lens than by the above described liquid crystal element.
Among the optical recording media that are proposed nowadays, amount of generation of the spherical aberration is large especially in a BD. Further, in the multilayer optical disc which has a plurality of recording layers, there is a possibility that the number of the recording layers becomes larger in future. That is, double layers disc is put in practical use now, but there is a possibility that four layers disc, six layers disc, and so on are put in practical use. In view of these situation, it is conceivable that optical pickup device which can correct much larger spherical aberration will become necessary in future.
Because of these, it is also conceivable that an optical pickup device that can correct large spherical aberration is structured by disposing a plurality of the liquid crystal elements which can perform correction of the spherical aberration that is provided with a plurality of the phase shift regions. However, it is not preferable because there is a possibility that wiring becomes complicated in such case. Especially in a case where an objective lens and a plurality of liquid crystal element are mounted in integrated manner on an actuator that is arranged to perform movement of the objective lens in order to suppress generation of coma aberration, problem of complicated wiring becomes severe.